Science Fiction films, despite their name, aren't known for scientific accuracy. So it should not be a big surprise if the writers overlooked what is likely to happen after the film is over even though the hero seems to have saved the world. After all, that's a problem for whoever writes the sequel. But here are a few cases where the aftermath would be so catastrophic there is no plausible basis for a sequel. Not that a sequel won't be produced anyway.
—David Foss